Characterization of the dTDP-rhamnose biosynthetic genes encoded in the rfb locus of Shigella flexneri.

The nucleotide sequence of the proximal half of the rfb region of Shigella flexneri has been determined, and the genes encoding enzymes involved in the biosynthesis of dTDP-rhamnose have been identified. These genes show strong homology to the rfb genes encoding dTDP-rhamnose biosynthesis in Salmonella enterica serovar typhimurium (strain LT2) and S. enterica serovar anatum (strain M32) (Jiang et al., 1991; Wang et al., 1992). An open reading frame upstream of rfbB was also identified which encoded a protein having strong similarity with GaIU, and has been designated galF. GalF has 92% amino acid sequence identity with an S. enterica LT2 gene, orf2X8, which is similarly situated upstream of rfbB (Jiang et al., 1991). The T7 expression system was utilized to identify proteins corresponding to those predicted from DNA sequence analysis. The similarity of the predicted proteins with proteins that are functionally identical or related, and with others of unknown function from the Yersinia enterocolitica O3 rfb region, and in the Escherichia coli K-12 rff region are also described. We have re-addressed the assignment of each gene of the dTDP-rhamnose pathway with the known enzymes of the pathway, in particular rfbC and rfbD. A reporter plasmid to detect genes encoding enzymes of the dTDP-rhamnose pathway is described. An analysis of the intergenic region between galF and rfbB has been made, and comparison with the same region from S. enterica LT2 discussed.